This invention relates to stand-off insulators supported by a frame to hold coiled wire electric heating elements. A typical use is to support such heating element in a plane, sometimes in parallel rows connected by reversing 180.degree. bends, so that both ends of a symmetrical insulator are employed.
Such insulators heretofore known are of several types. In a first type, the insulator tip or tips generally resembles an arrowhead, with hook-like projections on each side. When two adjacent turns of wire are pressed over the arrowhead projection, they are to spread apart and engage themselves behind the hook-like projections. In the other type, a throat is provided with a cross-slot; two adjacent turns of the coil are pressed together so that when released they are to spread apart toward the opposite ends of the slot. Both such types, as well as others, are shown in U.S. Pat. No. 4,363,959 to Cottrell et al.
Wire of types commonly used to form such coiled resistance elements may not be uniformly elastic, and further tends to lose elasticity with heating in use. Typically, heater element coils are formed by coiling with the successive turns of the coil in contact with each other; then tension is applied to their ends to stress the coils beyond their elastic limit and extend each to a desired length. This procedure does not assure that the turns of the coil will be equally spaced. Further, on assembly it is not practical to apply a fixed number of turns of the coil to the spacing of the stand-off insulators. One cannot therefore suspend the coiled wire with a chosen pre-tension between adjacent insulators; and any tension initially applied may be relieved on subsequent heating.
As a result of these factors, if a coil is mounted upon the arrowhead type insulator by adjacent turns whose spacing is greater than contemplated, its retention will not be secure. Similarly if the turns are spaced more closely than contemplated, they will be insecure if spaced on the throat-and-slot type insulator.
The spacing problem is compounded where the insulators are to be positioned not only in straight runs, but also adjacent to substantial angular bends of the heating coil. On the inside of a 180.degree. bend, adjacent coil turns should have their spacing decreased, while it should increase on the outside of the turn. In fact, the spacing at the turns has proved to be almost unpredictable. If initial tension does not draw the "straight run" portion of the coil tightly, both the inside and outside bends, measured along the end insulators, will tend to be less than 180.degree., so that two adjacent coil turns, intended to be held within the throat of an insulator, will tend to disengage by virtue of this departure from rectilinear alignment. On the other hand, excess tension in the coil will itself distort the angularity of the coil turn which (normally that turn closer to the 180.degree. bend) will be closer to its distorted position than would be contemplated by the design engineer.